Molded air cavity packages and methods for the production thereof

ABSTRACT

Molded air cavity packages and methods for producing molded air cavity packages are disclosed. In one embodiment, the molded air cavity package includes a base flange, retention posts integrally formed with the base flange and extending from the flange frontside in a direction opposite the flange backside, and retention tabs having openings through which the retention posts are received. A molded package body is bonded to the base flange and envelopes, at least in substantial part, the retention posts and the retention tabs. The molded air cavity package further includes package leads extending from the molded package body. In certain implementations, the package leads and the retention tabs comprise singulated portions of a leadframe. Additionally or alternatively, the retention posts may be staked or otherwise physically deformed in a manner preventing disengagement of the retention posts from the retention tabs along a centerline of the molded air cavity package.

TECHNICAL FIELD

Embodiments of the present invention relate generally to microelectronicpackaging and, more particularly, to molded air cavity packages andmethods for the production thereof.

Abbreviations

The following abbreviations appear throughout this document:

ACM—Molded Air Cavity or Air Cavity Molded;

Ag—Silver;

Au—Gold;

CTE—Coefficient of Thermal Expansion;

Cu—Copper;

IC—Integrated Circuit;

MEMS—Microelectromechanical Systems;

Mo—molybdenum;

PFPE—perfluoropolyether;

RF—Radio Frequency;

SiP—System-in-Package;

Wt %—Weight percent; and

° C.—degrees Celsius.

BACKGROUND

Air cavity packages are usefully employed to house semiconductor die andother microelectronic devices, particularly those supporting RFfunctionalities. Air cavity packages are fabricated in a variety ofdifferent manners, with different manufacturing approaches associatedwith varying benefits and drawbacks. In one common approach formanufacturing leaded air cavity packages, a discretely-fabricated piececommonly referred to as a “window frame” is bonded between the packageleads and a base flange. The window frame is produced from a dielectricmaterial, such as a ceramic, to provide electrical insulation betweenthe base flange and the package leads. The window frame may have arectangular planform geometry and a central opening, which defines anouter perimeter of the air cavity when the air cavity package isassembled. Prior to attachment of a lid or cover piece, one or moremicroelectronic devices are positioned within the air cavity and bondedto the base flange, which may serve as a heat sink and, perhaps, as anelectrically-conductive terminal of the package. Afterwards, themicroelectronic devices are electrically interconnected with the packageleads by, for example, wirebonding. The cover piece is then bonded overthe window frame to sealingly enclose the air cavity and completefabrication of the air cavity package.

In another air cavity package manufacturing approach, a molding processis carried-out to form a molded package body over and around selectedregions of the base flange and package leads in place of theabove-described window frame. Along with an exposed upper surface of thebase flange, the molded package body defines an open air cavity in whichone or more microelectronic devices are subsequently installed. Afterinstallation of the microelectronic devices and interconnection with thepackage leads, a cover piece is positioned in the air cavity and bondedto the molded package body to complete the package fabrication process.Utilizing such a mold-based manufacturing approach, so-called “moldedair cavity packages” can be fabricated in a manner similar to theabove-described, window frame-containing air cavity packages, but withgreater process efficiencies and at lower manufacturing costs. Theseadvantages notwithstanding, certain limitations continue to hamperprocesses for manufacturing molded air cavity packages, asconventionally performed. Such limitations may generally relate todifficulties encountered in maintaining precise alignment betweenpackage components prior to and/or through the molding process.Additionally, in the context of conventional molded air cavity packagefabrication processes, it may be difficult to ensure the formation ofreliable, high integrity seals between bonded components, as may becritical to preserve the sealed environment of the air cavity over thepackage lifespan in certain applications.

BRIEF DESCRIPTION OF THE DRAWINGS

At least one example of the present invention will hereinafter bedescribed in conjunction with the following figures, wherein likenumerals denote like elements, and:

FIGS. 1 and 2 are cross-sectional views of an ACM package, whichincludes multiple unique structural features and which is illustrated inaccordance with an exemplary embodiment of the present disclosure;

FIGS. 3-11 illustrate the ACM package shown in FIGS. 1-2, as shown atvarious stages of completion and fabricated in an accordance anexemplary ACM package fabrication process; and

FIGS. 12 and 13 are cross-sectional schematics of an exemplarycover-body interface usefully included in ACM package shown in FIGS.1-11, as illustrated prior to and after cover piece attachment,respectively.

For simplicity and clarity of illustration, descriptions and details ofwell-known features and techniques may be omitted to avoid unnecessarilyobscuring the exemplary and non-limiting embodiments of the inventiondescribed in the subsequent Detailed Description. It should further beunderstood that features or elements appearing in the accompanyingfigures are not necessarily drawn to scale unless otherwise stated. Forexample, the dimensions of certain elements or regions in the figuresmay be exaggerated relative to other elements or regions to improveunderstanding of embodiments of the invention.

DETAILED DESCRIPTION

The following Detailed Description is merely exemplary in nature and isnot intended to limit the invention or the application and uses of theinvention. The term “exemplary,” as appearing throughout this document,is synonymous with the term “example” and is utilized repeatedly belowto emphasize that the following description provides only multiplenon-limiting examples of the invention and should not be construed torestrict the scope of the invention, as set-out in the Claims, in anyrespect.

The term “air cavity package,” as appearing throughout this document,refers to a microelectronic package including a sealed cavity that is atleast partially filled with a gas, regardless of the internal pressurewithin the cavity. The “air cavity” of the air cavity package may beenclosed in an open air environment and, thus, may contain air atapproximately 1 atmosphere pressure with slight variations dependingupon elevation and processing temperatures during package manufacture.In other implementations, the “air cavity” of the air cavity package maybe enclosed in a partially evacuated chamber or a chamber containing aninert gas, such as argon, during manufacture and, thus, may not containpure air in all instances. The term “air cavity,” then, should beunderstood as referring to a gas-containing cavity, which may or may notbe partially evacuated and which is sealed from the ambient environment.Additionally, the seal formed between the air cavity and the ambientenvironment may not be hermetic, as strictly defined, but rather may bea low leakage seal having a gross leakage rate falling within acceptableparameters. Thus, as appearing herein, a cavity is considered “sealed”when little to no leakage (bubbles) are observed from the cavity'sexterior when the cavity is filled with air or another gas and the aircavity package is fully immersed in a liquid (e.g., PFPE) atapproximately 125° C. Finally, the term “molded air cavity package” andthe corresponding term “ACM package” refer to an air cavity package, aspreviously defined, and further including a package body principally orexclusively formed from one or more molded materials.

Overview

The following provides high performance ACM packages well-suited forhousing various types of microelectronic devices. Such microelectronicdevices include, but are not limited to, devices operable at relativelyhigh radio frequencies exceeding 100 megahertz and, perhaps, approachingor exceeding approximately 6 gigahertz in certain instances.Advantageously, embodiments of the below-described ACM packages areamenable to fabrication utilizing efficient, repeatable, cost-effectivemanufacturing processes. Such manufacturing processes may involve theformation of molded package bodies around leadframes and base flanges,which may be mechanically joined prior to molding. In one usefulapproach, the base flanges are fabricated to include raised locating orpiloting features, such as pillars or posts, which project upwardly fromthe upper principal surfaces or frontsides of the flanges and which arereceived through corresponding apertures provided in tab-like extensionsincluded in the leadframes (herein, “retention tabs”). The pillars orposts may then be deformed in a controlled manner, such as by staking,to mechanically capture the base flanges against the leadframes andthereby ensure proper component positioning leading into and/or throughthe molding process, particularly as the leadframe-flange assemblies areloaded into the mold tooling. The ACM packages may be manufactured on anindividual basis or, instead, fabricated in parallel by processing aleadframe array containing a relatively large number of interconnectedleadframes. In the latter instance, the above-described mechanicalcapture process can be performed globally across a leadframe array toattach a plurality of base flanges to their respective leadframes priorto molding.

After spatial alignment of the base flanges relative to the leadframes,and possible mechanical capture of the flanges against the leadframes,the molded package bodies of the ACM packages are formed. Due to thedesign flexibility afforded by molding, the molded package bodies can beimparted with various different geometries and structural features, astailored to best suit a particular application or package usage. Invarious embodiments, the molded package bodies may be formed toencompass or envelop any mechanical retention features located on thebase flanges and the leadframes; e.g., in the above-describedimplementation wherein the base flanges are fabricated to includeretention (e.g., staking) posts received through openings provided inthe leadframes (e.g., specifically, in the above-described retentiontabs), the molded package bodies may be formed to wholly envelope orcover corresponding post-tab pairs. Additionally, in the case of eachACM package, the molded package body may help define an open cavitythrough which a device mount area of the base flange is exposed, asviewed from the topside of the package prior to cover piece attachment.At least one microelectronic device is attached to the device mount areaof the base flange and appropriate interconnections are formed by, forexample, wirebonding. A lid or cover piece is then bonded to the upperperipheral edge portion or rim of the molded package body to sealinglyenclose the air cavity and complete fabrication of the ACM package.

In certain embodiments, an organic pressure-sensitive adhesive, such asa die attach material, may be utilized to attach the microelectronicdevices to the device mount areas of the base flanges. Alternatively, ametallic bonding material can be utilized. As a more specific example inthis latter instance, a low temperature sinter bond process may beusefully employed for device attachment purposes. In addition toproviding a highly robust metallurgical bond and diffusion at thedevice-flange interfaces in at least some instances, such a sinter bondprocess can be carried-out at relatively limited maximum processingtemperatures (e.g., peak processing temperatures less than 300° C.)through the application of elevated heat, controlled convergentpressures, or both heat and convergent pressures. As the sinteringprocess is carried-out at relatively low temperatures, materialconstraints placed on the package components are eased and issuesassociated with high temperature processing may be mitigated; e.g.,warpage or other structural degradation of the molded package bodiespotentially occurring at higher processing temperatures may be avoided.Furthermore, the resulting sintered bond layers can be produced withlittle to no voiding, controlled porosities, and highly controlledthicknesses to optimize performance parameters of the completed ACMpackage. Such sintered bond layers may contain lesser amounts of organicmaterials in implementations or, instead, may be essentially free oforganic materials; the term “essentially free of organic materials”defined herein as containing less than 1% organic materials, by weight.Examples of processes suitable for forming such sinter bond layers andpotential formulations of such layers are described in detail below.

Embodiments of the ACM package can be further imparted with an optimizedcover-body interface in addition to or in lieu of the other featureslisted above. The optimized cover-body interface is formed between thelower peripheral edge of cover piece and the upper peripheral edge orrim of the molded package body to which the cover piece is bonded. Asindicated by the term “optimized,” the cover-body interface includesunique structural features, which guide precision alignment of the coverpiece to the molded package body, which help ensure the formation ofhigh integrity bond at the cover piece-package body juncture, and/orwhich provide other benefits enhancing the package fabrication processand the ACM packages produced thereby. Examples of such features and,more generally, of an optimized cover-body interface are discussed morefully below in conjunction with FIGS. 12-13. First, however, anexemplary ACM package is discussed in conjunction with FIGS. 1-2, whilemethods for fabricating the exemplary ACM package (along with a numberof like ACM packages) are described below in conjunction with FIGS.3-11.

Non-Limiting Example of a Molded Air Cavity Package

FIGS. 1 and 2 are cross-sectional views of a leaded ACM package 20, asillustrated accordance with an exemplary embodiment of the presentdisclosure. While realized as a leaded package in the illustratedexample, ACM package 20 may assume other forms in alternativeimplementations, such as that of a no-lead package or a packagecontaining leads of a different type. Progressing from top to bottom inFIGS. 1-2, leaded ACM package 20 includes a lid or cover piece 22, aplurality of package leads 24, a molded package body 26, and a baseflange 28. Additionally, leaded ACM package 20 contains an air cavity30, which is bounded and defined by cover piece 22, molded package body26, base flange 28, and, to a lesser extent, package leads 24. Asindicated above, air cavity 30 may contain air, another inert gas, or agas mixture, and may or may not be partially evacuated or pressurizedrelative to the ambient environment. The hermicity of air cavity 30 willvary amongst embodiments, although ACM package 20 is usefully producedsuch that relatively little, if any leakage occurs between cavity 30 andthe ambient environment.

Molded package body 26 can be formed to have various differentgeometries and structural features. In the illustrated example, moldedpackage body 26 is formed to include a bottom edge portion or lowerperipheral skirt 32; the terms “lower,” “bottom,” and similar terms oforientation defined based upon proximity to the bottom principal surfaceor backside 34 of base flange 28. Lower peripheral skirt 32 is bonded toand extends around base flange 28, as taken about the centerline of ACMpackage 20; the centerline of ACM package 20 identified in FIG. 1 bydashed line 35 and extending substantially orthogonal to the upperprincipal surface or frontside 42 of base flange 28. As indicated inFIG. 2, lower peripheral skirt 32 may be formed as a continuous wall,which extends fully around the outer periphery of base flange 28. Infurther embodiments, lower peripheral skirt 32 may be formed as aninterrupted or discontinuous wall, as taken around the perimeter of baseflange 28; or molded package body 26 may be formed to omit such a lowerperipheral skirt.

The bottom principal surface or backside 34 of base flange 28 is exposedthrough a lower central opening, which is provided in molded packagebody 26 and which is peripherally bound by lower peripheral skirt 32. Byexposing flange backside 34 from the exterior or underside of ACMpackage 20 in this manner, mounting or attachment of ACM package 20within a larger system or device may be eased, while electricalconnection to flange backside 34 may be facilitated as may be usefulwhen, for example, flange 28 serves as a terminal of package 20. Asanother benefit, the exposed region of flange backside 34 may promoteheat removal from ACM package 20 by conductive heat transfer throughbase flange 28. The foregoing benefits are generally optimized when atleast a majority, if not the substantial entirety of flange backside 34(considered by surface area) is exposed through peripheral skirt 32 ofmolded package body 26, as shown. This notwithstanding, flange backside34 may not be exposed from the exterior of molded package body 26 oronly a relatively limited area of backside 34 may be externally exposedin alternative embodiments of package 20.

With continued reference to FIGS. 1-2, molded package body 26 alsocontains one or more inwardly-extending ledge portions, referred tohereafter as “lead isolation shelves 36.” Lead isolation shelves 36underlie the inner terminal end portions of package leads 24, whichextend into the package interior and to which device interconnectionsare formed. These terminal end portions of package leads 24 areidentified in FIGS. 1-2 by reference numeral “40” and are referred tohereafter as “proximal” lead ends; the term “proximal,” and the antonym“distal,” defined based upon relative proximity to package centerline35. Lead isolation shelves 36 extend from lower peripheral skirt 32 inan inward or inboard direction (that is, toward package centerline 35)and over an outer peripheral region of flange frontside 42. Leadisolation shelves 36 serve, in effect, as intervening dielectric layers,which reside between the respective lower surfaces of package leads 24and flange frontside 42, as taken vertically through ACM package 20along centerline 35. Lead isolation shelves 36 thus provide lead-flangeelectrical insulation, while further helping to mechanically joinpackage leads 24 and base flange 28. Finally, as indicated in FIGS. 1-2,lead isolation shelves 36 may surround the peripheral surfaces ofproximal lead end portions 40, while leaving exposed the upper surfacesof proximal lead end portions 40 from within the package interior forsubsequent electrical interconnection.

Molded package body 26 further includes an upper edge portion orperipheral rim 38, which is formed over proximal lead end portions 40opposite lower peripheral skirt 32. Upper peripheral edge portion 38extends around air cavity 30 and, in combination with cover piece 22,largely bounds or defines the periphery of cavity 30. In addition toproviding a controlled vertical separation or standoff between coverpiece 22 and package leads 24 along package centerline 35, upperperipheral edge portion 38 also serves as a physical interface formating engagement with cover piece 22. Accordingly, upper peripheraledge portion 38 may be imparted with a planform shape and dimensionsgenerally corresponding with the planform shape and dimensions of alower peripheral edge 44 of cover piece 22. Additionally, upperperipheral edge portion 38 and/or lower peripheral edge 44 may beimparted with certain features facilitating cover piece attachment andthe formation of a high integrity bond between cover piece 22 and moldedpackage body 26, as described below. Alternative embodiments of moldedpackage body 26 may lack upper peripheral edge portion 38, which may bereplaced by another, discretely-fabricated structure (e.g., a windowframe) or may be rendered unneeded by direct bonding of lower peripheraledge 44 to proximal lead end portions 40. Generally, however, coverpiece attachment can be enhanced through the provision of such an upperperipheral edge portion, which is advantageously formed with lowerperipheral skirt 32 and lead isolation shelves 36 as a single,integrally-formed molded structure or body.

ACM package 20 can contain any number and type of microelectronicdevices, which can be interconnected as appropriate to yield, forexample, an SiP. Such microelectronic devices can include IC-carryingsemiconductor die, MEMS die, optical sensors, and passive devices, suchas discrete inductors, resistors, diodes, and capacitors, to list but afew examples. In the relatively simple example shown in FIGS. 1-2, ACMpackage 20 contains a single microelectronic device 50, such an RF powertransistor die or another IC-bearing semiconductor die. Microelectronicdevice 50 includes a lower surface or backside, which is attached toflange frontside 42 by at least one device bond layer 52.

Device bond layer 52 can be composed of a die attachment material, suchas an epoxy and a pressure-sensitive adhesive, in an embodiment.Alternatively, device bond layer 52 may be formed from a metallic-basedbonding material; that is, a bonding material predominately composed ofone or more metallic constituents, by weight. In certain embodiments,device bond layer 52 is formed utilizing a low temperature sinteringprocess in which metal particles (e.g., Cu, Ag, and/or Au particles inthe nanometer or micron size range) are densified to form the desiredbond layer. In such embodiments, device bond layer 52 may consistessentially of metallic materials; or, instead, may contain lesseramounts of non-metallic constituents, such as one or more organicmaterials added to enhance targeted bond layer properties. If desired,and as exclusively shown in FIG. 1, a containment layer or bead 60 mayalso be formed around the outer periphery of device bond layer 52 whencomposed of metallic material, such as a Ag-based material, prone tomigration. Sinter containment bead 60 can be produced from an epoxy oranother polymeric material suitable for usage as a dam or blockadefeature, which physically impedes or blocks undesired migration ofdevice bond layer 52 (when formed as a sintered bond layer). Lowtemperature sintering processes suitable for forming device bond layer52 are discussed more fully below in conjunction with FIG. 11.

With continued reference to FIGS. 1-2, base flange 28 can be any body ofmaterial, layered structure, or composite structure serving as asubstrate or carrier supporting microelectronic device 50. Base flange28 may serve as an electrically-conductive terminal of ACM package 20and, perhaps, as a heat sink or heat spreader. Base flange 28 may thusassume the form of a monolithic metallic structure, plate, slug, or thelike in certain implementations. In other implementations, base flange28 may itself assume the form of a printed circuit or wiring board. Baseflange 28 may be produced from, for example, an organic material (e.g.,a resin similar or identical to that from which printed circuit boardsare produced) containing metal (e.g., Cu) coining. In other embodiments,base flange 28 may have a multilayer metallic construction; e.g., baseflange 28 may contain multiple thermally-conductive layers, which arebonded in a stacked or laminated arrangement. Base flange 28 willtypically be composed predominately of one or more metals havingrelatively high thermal conductivies, such as Cu. As a more specificexample, in an embodiment wherein base flange 28 is a layered orlaminated structure, base flange 28 may include at least one Cu layercombined with at least one disparate metal layer having a CTE less thanthat of the Cu layer. The disparate metal layer may be composed of, forexample, Mo, a Mo—Cu alloy, or a Mo—Cu composite material. In thismanner, base flange 28 may be imparted with both a relatively highthermal conductivity and a lower effective CTE, which is more closelymatched to that of microelectronic device 50 and/or to that of moldedpackage body 26. Thermally-induced stress within ACM package 20 may befavorably reduced as a result.

Circuitry may be formed on the frontside of microelectronic device 50,along with a number of bond pads 54. As shown exclusively in FIG. 1,bond pads 54 can be electrically interconnected with proximal lead endportions 40 utilizing, for example, a number of wire bonds 56. Inalternative implementations, ACM package 20 may be a no-lead package oranother interconnection approach may be employed to electricallyinterconnect bond pads 54 of device 50 with corresponding packageterminals. In one embodiment, a first lead 24(a) projects from a firstside of ACM package 20 and serves as an input lead electrically coupledto the input (e.g., gate) terminal of microelectronic device 50; while asecond lead 24(b) projects from a second, opposing side of package 20and serves as an output lead electrically coupled to the output (e.g.,drain) terminal of device 50. In certain instances, base flange 28 mayitself serve as a ground reference terminal of ACM package 20 and,therefore, may be electrically coupled to a source terminal of device50. Again, while shown as containing a single microelectronic device 50in FIGS. 1-2, ACM package 20 can contain any number and type ofmicroelectronic devices operably coupled utilizing variousinterconnection schemes. For example, in a further implementation, ACMpackage 20 may contain two or more microelectronic device 50electrically coupled in series or parallel between any practical numberof package leads 24(a)-(b).

After installation of microelectronic device 50, and interconnection ofdevice 50 with package leads 24, cover piece 22 is positioned overmolded package body 26 and bonded to upper peripheral edge portion 38 tosealingly enclose air cavity 30. Lower peripheral edge 44 of cover piece22 may be bonded to upper peripheral edge portion 38 of molded packagebody 26 by a ring of bonding material 46, which is referred to hereafteras “cover bond layer 46.” Cover bond layer 46 can be composed of anymaterial or materials suitable for mechanically attaching cover piece 22to molded package body 26, while forming a gas-tight or substantiallygas-tight seal around the periphery of the cover-body interface. Forexample, cover bond layer 46 may be composed of a metallic-based bondingmaterial, which may be formed utilizing a low temperature sinteringprocess similar to that mentioned above and described more fully belowas advantageously utilized to produce device bond layer 52.Alternatively, cover bond layer 46 can be formed from a dispensed (e.g.,high temperature) epoxy, a B-stage epoxy, or another die attachmentmaterial. An exemplary process for fabricating ACM package 20 will nowbe described in conjunction with FIGS. 3-13.

Examples of Air Cavity Package Fabrication Methods

Whether produced on an individual basis or in parallel with a number ofother ACM packages, ACM package 20 is conveniently fabricated utilizinga leadframe-based manufacturing approach. In particular, ACM package 20may be manufactured to incorporate a leadframe, which contains packageleads 24 and other physically-interconnected features, at least some ofwhich may be removed during the course of ACM package fabrication. Anexample of a leadframe 62 suitable for usage in the manufacture of ACMpackage 20 is shown in FIG. 3. In this example, leadframe 62 isfabricated as a relatively thin strip or plate-like body composed of ametallic material, such as Cu or a Cu-based alloy. The body of leadframe62 is machined (e.g., stamped), etched, laser cut, or otherwiseprocessed to define the various leadframe features of leadframe 62. Inaddition to package leads 24, these features include a central opening64; a number of connective fingers or spars (herein, “dam bars 66”), aplurality of retention tabs 68, and an outer peripheral leadframeportion 72. Dam bars 66 join package leads 24 and retention tabs 68 tothe plate-like body of leadframe 62. Additionally, dam bars 66 mayfacilitate handing and positioning of leadframe 62 leading into and,perhaps, through the molding process. After molding, dam bars 66 may besevered and removed along with other sacrificial leadframe portions,such as outer peripheral leadframe portion 72. Outer peripheralleadframe portion 72 may likewise include various openings or cutoutsfacilitating handling of leadframe 62 during ACM package fabrication.

Retention tabs 68 extend laterally inwardly of package leads 24; thatis, toward the longitudinal axis of leadframe 62 represented in FIG. 3by dashed line 70. Notably, retention tabs 68 are formed to extend, atleast in part, in a different horizontal plane than does the remainderof leadframe 62 including package leads 24; the term “horizontal plane,”as appearing herein, referring to a plane orthogonal to packagecenterline 35 (FIG. 1) and, perhaps, parallel to flange frontside 42(FIGS. 1-2). Retention tabs 68 thus provide a vertical separation orstandoff between base flange 28 (FIGS. 1-2) and leadframe 62 when placedin contact therewith. This vertical standoff is shown more clearly inFIG. 7 (addressed below) and enables the formation of lead isolationshelves 36 during molding. The inner terminal ends of retention tabs 68are also imparted with openings, through holes, or apertures. Theseopening serve as locating or registration features, which receivecorresponding raised locating features, such as staking posts of thetype described below, to precisely position base flange 28 relative toleadframe 62 and, particularly, relative to package leads 24. Furtherdescription in this regard is set-forth below in conjunction with FIGS.5-8. In further embodiments, leadframe 62 can be fabricated to omit suchfeatures and/or to contain other structural features in addition tothose shown in FIG. 3, such as additional sets of package leads.

If desired, ACM package 20 may be fabricated as a discrete unit byindividually processing leadframe 62 as a pre-singulated structure.However, process efficiencies will typically be increased andmanufacturing costs lowered by manufacturing ACM package 20 in parallelwith a relatively large number of similar ACM packages. In this regard,ACM package 20 may be produced in parallel with other, non-illustratedACM packages by concurrently processing a plurality of leadframesinterconnected as a leadframe array. Such a leadframe array can containrelatively large number of leadframes arranged in, for example, a twodimensional grid layout or a linear strip layout. By way of example,FIG. 4 illustrates a limited portion of a leadframe array 74, which hasa strip-like form factor and which contains leadframe 62 along with anumber of other leadframes. As can be seen, the illustrated portion ofleadframe array 74 contains leadframe 62 along with two additionalleadframes 76, 78 (partially shown) adjoined to opposing edge portionsof leadframe 62. For clarity, dashed lines 80 generally demarcate theboundaries between leadframes 62, 76, 78 in FIG. 4. Leadframe array 74may be considerably larger than the illustrated portion and can containany practical number of interconnected leadframes, which aresubsequently singulated (e.g., by sawing, water jetting, laser cutting,or the like) at a later juncture during package fabrication. Thebelow-described process steps can thus be performed globally across aleadframe array, such as leadframe array 74 (FIG. 4), to manufacture aplurality of additional ACM packages in parallel with ACM package 20when desired.

Advancing to FIGS. 5-6, base flange 28 is next positioned with respectto leadframe 62 utilizing, for example, a pick-and-place tool. Baseflange 28 can be positioned with respect to leadframe 62 by positioningor movement of flange 28, movement of leadframe 62, or a combinationthereof. As a more specific example, and referring to the exemplaryorientation of base flange 28 and leadframe 62 shown in the drawingfigures, base flange 28 may be placed on a temporary support structure,carrier, or fixture; and leadframe 62 may be lowered onto flange 28 toachieve the desired positioning. Such an approach may be useful when aplurality of ACM packages are manufactured in parallel. In this case, anappropriate number of base flanges can be distributed across such acarrier or fixture, and a leadframe array (e.g., array 74 shown in FIG.4) may then be lowered into its desired position to concurrentlyposition the interconnected leadframes relative to the array of baseflanges. Alternatively, the illustrated orientation of base flange 28and leadframe 62 may be inverted, and base flange 28 may be lowered ontoleadframe 62 during flange-leadframe positioning. In either case, asindicated in FIG. 5 by arrows 82, frontside 42 of base flange 28 ispositioned against the underside of leadframe 62 and, specifically,placed in direct or intimate physical contact with retention tabs 68.

As previously indicated, base flange 28 is advantageously produced tocontain certain dedicated locating or piloting features, whichphysically register or pilot with corresponding features provided onleadframe 62. Examples such locating features are shown in FIG. 6, whichdepicts base flange 28 as including a number of raised pillars or posts84 (herein, “retention posts 84”). Retention posts 84 project upwardlyfrom frontside 42 of base flange 28 and are received through theopenings or apertures provided in retention tabs 68. Base flange 28 isproduced to include a number of retention posts 84 equivalent to theretention tab count, with the spatial distribution, shape, and outerdiameters of posts 84 generally matching the distribution, shape, andinner diameters of the openings provided in retention tabs 68. Forreasons of mechanical integrity, part count reduction, and/or processefficiency, retention posts 84 are usefully formed as integral featuresof base flange 28, rather than as discrete features subsequently bondedto or otherwise joined to flange frontside 42. In one approach,retention posts 84 are produced utilizing a punching operation duringwhich localized regions of base flange 28 are deformed upwardly (towardflange frontside 42) by application of a controlled, localized strikeforce to selected regions of flange backside 34. Correspondingly, such apunching operation may also form recesses, blind tunnels, or divots 86in backside 34 of flange 28, as shown most clearly in FIG. 5. Onceformed, divots 86 can also be leveraged for alignment or handlingpurposes and engaged by a specialized fixture in embodiments of the ACMpackage fabrication method.

In certain implementations, base flange 28 may be fabricated to includeat least one texturized surface are or region. As appearing herein, asurface region is considered “texturized” when imparted with anon-planar surface topology having an average feature height or depthexceeding 1 μm. In this case, base flange 28 can be imparted with anynumber of texturized surface regions, which are advantageously formed atlocations contacted by molded package body 26 (FIGS. 1-2) to increasethe integrity of the mechanical bond formed at the molded packagebody-flange interfaces. In the illustrated example, specifically, baseflange 28 is fabricated to include a peripheral knurled surface region88, which extends over an outer peripheral portion of flange frontside42 and which may be imparted with a ring-shaped planform geometry.Knurled surface region 88 extends around device mount area 58 and maythus underlie proximal end portions 40 of package leads 24 and leadisolation shelves 36 when ACM package 20 is completed, as consideredwhen progressing along centerline 35 from cover piece 22 toward flangebackside 34. Consequently, in such embodiments, lead isolation shelves36 may intimately contact and be directly bonded to knurled surfaceregion 88 following formation of molded package body 24, as describedbelow in conjunction with FIGS. 9-11. Finally, as shown in FIG. 6,knurled surface region 88 may be formed adjacent the junctures betweenretention posts 84 and flange frontside 42 such that posts 84 generallyextend from surface region 88 in an upward direction opposite frontside42. In further embodiments, selected surface regions of base flange 28may be texturized in a different manner (e.g., the outer perimeter offlange 28 may be knurled or otherwise texturized), or base flange 28 maylack any such texturized regions.

Continuing with the exemplary ACM package fabrication process, in atleast some implementations, base flange 28 is next mechanically capturedagainst leadframe 62. In this regard, process steps may now be performedto physically retain base flange 28 and leadframe 28 in their desired,intimately-contacting positions, thereby preventing flange-leadframeseparation along centerline 35 in a direction away from leadframe 62;that is, in a direction opposite the insertion axis generally parallelto arrows 82 shown in FIG. 5 and along which posts 84 are inserted intothe openings provided in retention tabs 68. In various embodiments, thismay be accomplished by deforming, in a controlled manner, the terminalends of the raised locating features (posts 84 in the illustratedexample) after positioning base flange 28 against leadframe 62. Incertain implementations, a crimping or bending operation can becarried-out to physically capture base flange 28 against leadframe 62.In other embodiments, a staking process may be carried-out to laterallyexpand the outer terminal ends or “staking heads” of posts 84 to preventflange-leadframe disengagement. When staked or intended for staking inthis manner, the more specific term “staking post” rather than the moregeneric term “retention post” may be utilized in reference to posts 84.By way of non-limiting example, additional description of a stakingprocess suitable for mechanically capturing base flange 28 againstleadframe 62 will now be provided in conjunction with FIGS. 7-8.

FIG. 7 illustrates, in step-by-step increments, a staking operationsuitably performed to physically deform one of retention posts 84 afterinsertion through a corresponding retention tab opening provided inleadframe 62. As indicated on the left side of FIG. 7, a staking tool 90is brought into contact with a head portion 92 of retention post 84.Sufficient pressure is applied to circumferentially-expand or splay-outhead portion 92, as further shown on the right side of FIG. 7. Theresultant structure is also shown from an isometric perspective view inFIG. 8. Heat may or may not be applied during the staking process, whichmay be repeated on an iterative basis to circumferentially-expand thehead portions of the other retention posts 84 shown in FIG. 6. Bycircumferentially-expanding head portion 92 of each retention post 84 inthis manner, each head portion 92 may be physically prevented frompassing through its corresponding opening in retention tab 68 and baseflange 28 may be captured against leadframe 62. Precise alignment alonga horizontal plane (orthogonal to the plane of the page in FIG. 7)between base flange 28 and leadframe 62 may therefore be ensured, whilevertical leadframe-flange separation is prevented leading into and/orthrough the subsequently-performed molding operation. Following staking,the expanded head portion 92 may feature a depression or cavity, whichmay have a generally frustoconical shape, as shown in phantom line onthe right side of FIG. 7 and further shown in FIG. 8.

Turning to FIGS. 9-10, the molding process in next carried-out to createmolded package body 26 over and around selected regions of leadframe 62and base flange 28. As shown most clearly in FIG. 9, molded package body26 is produced to include an open upper end such that air cavity 30, notyet enclosed by cover piece 22, opens in an upward direction away frombase flange 28. As noted above, molded package body 26 is formed so asto leave exposed device mount area 58 of base flange 28 and proximallead end portions 40 of package leads 24. In contrast, molded packagebody 26 may wholly envelope retention tabs 68 and retention posts 84(FIGS. 7-8) such that tabs 68 and posts 84 are buried within packagebody 26 and therefore hidden from view from the package exterior. Atsome juncture following the molding process, selected portions ofleadframe 62 may be severed or trimmed away including, for example, dambars 66 and outer peripheral leadframe portion 72 identified in FIGS.3-4. Additionally, the outer end portions of retention tabs 68 (that is,the ends of tabs 68 adjacent dam bars 66 as best seen in FIGS. 3-4) maybe severed such that tabs 58 are imparted with singulated surfacesvisible from an exterior of molded package body 26 in the completedpackage. Leadframe singulation or trimming therefore results electricalisolation of package leads 24, which may otherwise be electricallybridged to base flange 28 by dam bars 66 and retention tabs 68. As dambars 66 have been severed and partially or wholly trimmed away,retention tabs 68 are no longer physically and electrically coupled topackage leads 24 through dam bars 66. Processes suitable for singulatingor trimming leadframe 62 include, but are not limited to, sawing, lasercutting, water jetting, stamping, scribing (with or without punching),and routing.

At this juncture in the fabrication process, ACM package 20 is now nearcompletion. The remaining principal process steps remaining for ACMpackage completion are device installation, interconnection formation,and cover piece attachment. The physical features associated with suchbackend process steps are shown in FIG. 11, which is an explodedisometric view of package 20 in a partially-completed state. As package20 is shown in partially-completed state in FIG. 11, a prime symbol (′)is appended to the end of reference numeral “20” and when referring tothe partially-completed package below. In certain cases, the backendprocess steps may be performed by an entity distinct from that whichperformed the previously-described frontend process steps. In thisregard, a first entity (herein, the “supplier”) may produce ACM package20′ in its partially-completed state shown in FIG. 11, but lackinginstalled devices, electrical interconnections, or bonded cover pieces.The supplier may then ship or otherwise provide partially-completed ACMpackage 20′ (along with other partially-completed ACM packages andmating cover pieces) to a second entity (herein, the “customer”). Thecustomer may then perform the device installation, interconnectionformation, and cover piece attachment processes at a subsequent point intime. In still other instances, a single entity may perform all processsteps involved in ACM package fabrication or the various ACM packagefabrication process steps may be divided amongst multiple entities in adifferent manner.

Continuing with the exemplary ACM package fabrication process, one ormore microelectronic device are now installed within air cavity 30 ofACM package 20′ (FIG. 11). Device attachment can be performed utilizingany suitable adhesive or bonding material including organicpressure-sensitive adhesives, such commercially-available die attachmaterials. Alternatively, device attachment may be conducted utilizing ametallic-based bonding process, such as a low temperature sinteringprocess of the type briefly mentioned above and described more fullybelow. To produce such sintered bond layer(s), one or more layers ofmetal particle-containing precursor material may be obtained, applied tothe appropriate regions of ACM package 20′, and then processed totransform the bond layer precursor material into the sintered bondlayer(s). Examples of low temperature sintering processes suitable forforming one or more sintered bond layers, and specifically device bondlayer 52 shown in FIGS. 1-2, will now be described.

When a low temperature sinter bond process is utilized to produce devicebond layer 52 (FIGS. 1-2), a metal particle-containing precursor layer52′ (FIG. 11) may first be applied at the interface of microelectronicdevice 50 and device mount area 58 of base flange 28. Precursor layer52′ can be applied to the backside of device 50, which is thenpositioned (e.g., utilizing a pick-and-place tool) in its desiredlocation on flange frontside 42. Alternatively, precursor layer 52 maybe applied directly to device mount area 58, and microelectronic device50 may then be placed or seated thereupon. Generally, precursor layer52′ may be applied as a patterned or continuous layer having a planformshape and dimensions roughly corresponding to that of microelectronicdevice 50. Metal particle-containing precursor layer 52′ can be appliedutilizing a wet state or dry state application technique of the typedescribed below, and a low temperature sintering process may then becarried-out. Additional description of precursor material applicationand sintering processes suitable for usage in the fabrication of ACMpackage 20 will now be discussed in detail. Further description of suchprocesses can also be found in the following co-pending U.S. patentapplications, which are further incorporated by reference: U.S. patentapplication Ser. No. 15/269,629, filed with the United States Patent andTrademark Office on Sep. 19, 2016, and entitled “AIR CAVITY PACKAGES ANDMETHODS FOR THE PRODUCTION THEREOF”; and U.S. patent application Ser.No. 15/363,671, filed with the United States Patent and Trademark Officeon Nov. 29, 2016, and entitled “MICROELECTRONIC MODULES WITHSINTER-BONDED HEAT DISSIPATION STRUCTURES AND METHODS FOR THEFABRICATION THEREOF.”

Wet state application techniques suitable for applying metalparticle-containing precursor layer 52′ include, but are not limited to,screen or stencil printing, doctor blading, spraying, dipping, and fineneedle dispense techniques. When a wet state application technique isemployed, a flowable or wet state coating precursor material isinitially obtained by, for example, independent production or purchasefrom a third party supplier. In addition to metal particles (describedbelow), the wet state coating precursor material contains otheringredients (e.g., a solvent and/or surfactant) to facilitate wet setapplication, to adjust the viscosity of the precursor material, toprevent premature agglomeration of the metal particles, or to serveother purposes. In one embodiment, the wet state coating precursormaterial contains metal particles in combination with a binder (e.g., anepoxy), a dispersant, and a thinner or liquid carrier. The volume ofsolvent or liquid carrier contained within the coating precursormaterial can be adjusted to tailor of the viscosity of the precursormaterial to the selected wet state application technique. For example,in embodiments wherein the precursor material is applied by screenprinting or doctor blading, the coating precursor material may containsufficient liquid to create a paste, slurry, or paint. After applicationof the wet state coating material, a drying process can be carried-outto remove excess liquid from the metal particle-containing precursormaterial, if so desired.

In further embodiments, metal particle-containing precursor layer 52′can be applied utilizing a dry state application technique. For example,a film transfer process can be employed to apply precursor layer 52′ tothe appropriate surfaces of base flange 28 or microelectronic device 52.In this regard, a dry film may first be prepared by, for example,initially depositing (e.g., screen printing or otherwise dispensing) oneor more metal particle-containing precursor layer 52′ onto a temporarysubstrate or carrier, such as a plastic (e.g., polyethyleneterephthalate) tape backing. The metal particle-containing precursorlayer 52′ may be applied to the carrier in a wet, flowable state andthen heated or otherwise dried to yield a dry film, which is transferredto the appropriate package component surfaces. Heat, pressure, or bothheat and pressure are then applied to adhere the metalparticle-containing precursor layer (dry film) to the appropriatecomponent surfaces. The carrier (e.g., tape backing) may then be removedby physical removal (e.g., peeling away) or by dissolution in a chemicalsolvent. In still further embodiments, one or more freestanding filmsmay simply be positioned between the air cavity package componentsduring the stacking or build-up process (also considered “film transfer”in the context of this document) by, for example, positioning afreestanding film over base flange 28 prior to placement of device 50.

The metal particles dispersed within metal particle-containing precursorlayer 52′ can have any composition, shape, and size enabling theparticles to form a substantially coherent adhesive layer pursuant tothe below-described sintering process. In one embodiment, metalparticle-containing precursor layer 52′ contains Au, Ag, or Cuparticles, or a mixture thereof. In another embodiment, the metalparticles contained within the precursor layer 52′ consist essentiallyof Ag or Cu particles. The metal particles contained within precursorlayer 52′ may or may not be coated with an organic material. Forexample, in some implementations, the metal particles may be coated withan organic dispersant, which prevents physical contact between theparticles to inhibit premature agglomeration or particle sintering. Whenpresent, any such organic particle coating may be burned away orthermally decomposed, whether in whole or in part, during thebelow-described metal sintering process. In still further embodiments,other material systems amenable to low temperature sintering, whethercurrently known or later developed, may be utilized in the ACM packagefabrication process.

The metal particles contained within precursor layer 52′ can have anyshape or combination of shapes including, but not limited to, sphericalshapes, oblong shapes, and platelet or laminae shapes. The averagedimensions of the metal particles will vary in conjunction with particleshape and process parameters. However, in general, the average maximumdimension of the metal particles (e.g., the diameter of the metalparticles when spherical or the major axis of the metal particles whenoblong) may be between about 100 microns (μm) and about 10 nanometers(nm) in an embodiment. In other embodiments, the metal particles mayhave average maximum dimension greater than or less than theaforementioned range. In certain implementations, a mixture of metalparticles having average maximum dimensions in both the nanometer andmicron range may be present within the precursor material. In otherimplementations, only nanoparticles (that is, particles having averagemaximum dimension between 1 and 1000 nm) may be contained within metalparticle-containing precursor layer 52′. As a specific, albeitnon-limiting example, precursor layer 52′ may contain at least one ofAg, Au, or Cu nanoparticles or micron-sized particles in an embodiment,with Ag or Cu nanoparticles being preferred.

After application of metal particle-containing precursor layer 52′ andplacement of microelectronic device 50, a low temperature sinteringprocess is performed to produce a sintered bond layer forming, in thisexample, device bond layer 52 (FIGS. 1-2). The low temperature sinteringprocess can be carried-out under any process conditions suitable fortransforming precursor layer 52′ into a sintered bond layer, noting thatsome diffusion may occur from precursor layer 52′ into base flange 28.The sinter bond process thus advantageously forms low stress,mechanically-robust, solid state metallurgical diffusion bonds at thebond joint interfaces. The sintering process may be performed with orwithout pressure, with or without heating (although some degree ofelevated heat will typically be applied), and in any suitable atmosphere(e.g., open air or in the presence of an inert gas, such as nitrogen).As a point of emphasis, the sintering process is carried-out at maximumprocessing temperatures (T_(MAX)) less than the melt point of the metalparticles contained within the precursor layer 52′. Indeed, in manyembodiments, T_(MAX) will be significantly less than the melt point ofthe metal particles and, perhaps, less than one half the melt point ofthe particles considered on an absolute temperature scale (in Kelvin).Generally, T_(MAX) will be greater than room temperature (considered 20°C. herein) and less than 300° C. Comparatively, the melt point of Ag,Au, and Cu particles in a nanometer or micron size range will commonlyrange between approximately 950° C. to 1100° C. To provide a stillfurther example, T_(MAX) may be between approximately 170° C. and 300°C. in an embodiment. In still further embodiments, T_(MAX) may begreater than or less than the aforementioned range, providing thatT_(MAX) (in conjunction with the other process parameters) is sufficientto induce sintering of the metal particles without liquefaction of themetal particles.

A multistage heating schedule can be employed during the sinteringprocess. In this case, the multistage heating schedule may entailheating partially-fabricated ACM package 20′ (and any other ACM packagesproduced in parallel therewith) when the air cavity packages areproduced utilizing a panel-level fabrication process such as thatpreviously described) to a first temperature (T₁) less than T_(MAX) fora first time period, gradually increasing or ramping-up the temperatureprocess to T_(MAX), and then maintaining T_(MAX) for a second timeperiod. A cool down period may follow. In one embodiment, and by way ofnon-limiting example only, T₁ may range from approximately 100 to 200°C., while T_(MAX) is greater than T₁ and ranges from approximately 170to 280° C. As discussed below, the process parameters employed may ormay not be controlled to fully decompose any organic material from metalparticle-containing precursor layer 52′ during the sintering process.

In at least some implementations of the ACM package fabrication method,a controlled convergent pressure or compressive force is applied acrossthe partially-fabricated air cavity packages during the sinteringprocess. When applied, the convergent pressure can be delivered as asubstantially constant force or, instead, varied in accordance with atime-based or temperature-based schedule. Any suitable mechanism can beutilized to apply the desired convergent pressure including bulkweights, resilient bias devices (e.g., spring-loaded plungers or pins),clamps, hydraulic presses, and the like. The pressure applied may beselected based upon various factors including the desired finalthickness of the sintered bond layer, the desired porosity of thesintered bond layer, and the composition of metal particle-containingprecursor layer 52′. In one embodiment, and by way of non-limitingexample only, a maximum pressure (P_(MAX)) ranging between about 0.5 andabout 20 megapascal is applied during the sintering process. In otherembodiments, P_(MAX) may be greater than or less than the aforementionedrange, if pressure is applied during the sintering process.

As indicated briefly above, the sintered bond layer produced pursuant tothe above-described metal sintering process may be composedpredominately of one or more sintered metals. Again, the sintered bondlayer may or may not contain organic materials. In one embodiment, thesintered bond layer consist essentially of one or more metals (e.g.,essentially pure Cu or essentially pure Ag) and are essentially free oforganic material; that is, contain less than 1 wt % of organicmaterials. In other embodiments, the sintered bond layer may containresin or other organic fillers. For example, in another implementation,the sintered bond layer may contain organic materials that increasepliability, such as an epoxy, to reduce the likelihood of crackformation and propagation across thermal cycles. Depending upon thedesired final composition of the sintered bond layer, the parameters ofthe sintering process may be controlled to decompose organic materialsfrom metal particle-containing precursor layer 52′, in whole or in part.Additionally, the sintered bond layer may be produced to have a desiredporosity, which may range from 0% to 30% by volume in an embodiment. Inanother embodiment, the sintered bond layer may be formed to each have aporosity of less than 1% by volume. Finally, the thickness of thesintered bond layers will vary amongst embodiments, but may rangebetween about 5 μm and about 100 μm and, preferably, between about 15 μmand about 35 μm in one exemplary and non-limiting embodiment. In anotherembodiment wherein the sintered bond layers are composed of essentiallypure Ag or Cu, the sintered bond layer may have a thickness range fromabout 40 μm to about 100 μm.

Following device attachment, appropriate electrical interconnections arenext formed between the installed microelectronic device(s) and theterminals exposed from within the package interior. In the case ofexemplary package 20′, specifically, ball bonding or another wirebondingprocess is conveniently performed to form wirebonds 56 electricallycoupling bond pads 54 of microelectronic device 30 to the exposed uppersurfaces or “wirebond shelves” of proximal end portions 40 of packageleads 24, as shown in FIG. 1. In other embodiments, a differentinterconnection technique can be employed. Similar inter-deviceconnections may also be formed in embodiments in which ACM package 20(FIGS. 1-2) contains multiple microelectronic devices, which areinterconnected form an SiP. Electrical testing may further be performed,if desired, following device attachment and prior to the below-describedcover piece attachment operation.

To complete partially-fabricated ACM package 20′ (FIG. 11) and yield thecompleted version of package 20 shown in FIGS. 1-2, cover piece 22 isattached to upper peripheral edge portion 38 of molded package body 26.In certain instances, a low temperature sintering process can beutilized to join cover piece 22 to molded package body 26; noting that,in such embodiments, the foregoing description pertaining to device bondlayer 52 is equally applicable to cover bond layer 46 shown in FIGS.1-2. Alternatively, a flowable adhesive material, such as a hightemperature epoxy or other die attachment material, may be utilized tobond cover piece 22 to molded package body 26. Bond layer 46, upperperipheral edge portion 38 of molded package body 26, and lowerperipheral edge 44 of cover piece 22 cooperate to sealingly enclose aircavity 30 after device installation and electrical interconnection.Finally, if not yet performed, leadframe 62 can be singulated or trimmedto complete ACM package fabrication. As indicated above, embodiments ofACM package 20 are advantageously produced to an “optimized” body-coverinterface having certain features, which aid the cover piece attachmentprocess and help to ensure the reliable formation of a continuous, highintegrity bond joint or seal at the interface between upper peripheraledge portion 38 of molded package body 26 and lower peripheral edge 44of cover piece 22. Additional description of an exemplary cover pieceattachment process and certain features that may facilitate cover pieceattachment will now be described in conjunction with FIGS. 12-13.

Examples of Features Enhancing Bond Performance at the Cover-BodyJuncture

FIGS. 12 and 13 are cross-sectional schematics of an exemplarycover-body interface usefully included in ACM package 20 shown in FIGS.1-11, as illustrated prior to and after cover piece attachment 22,respectively. As indicated above, the term “cover-body interface” refersto the physical features formed at and adjacent the juncture betweenlower peripheral edge 44 of cover piece 22 and upper peripheral edgeportion 38 of molded package body 26. In the illustrated example, thiscover-body interface is optimized through the inclusion of certainunique physical features. Such features may guide precision alignment ofcover piece 22 relative to the remainder of package 20 during coverpiece attachment; help promote the reliable formation of a highintegrity, peripherally-continuous, low leakage bond at the cover-bodyjuncture; and/or provide other desirable benefits. In the exemplaryembodiment shown in FIGS. 12-13, specifically, the illustratedcover-body interface includes three principal types of features: (i)hardstop features 96, 98; (ii) an annular channel, void, or groove 100;and (iii) a number of angled contact surfaces 102, 104. Each of thesefeature are described below, in turn, with the understanding that suchfeatures can vary in type, number, and disposition in alternativeembodiments of ACM package 20. For example, while certain features, suchas annular channel 100, are shown as predominately or exclusively formedin upper peripheral edge portion 38 of molded package body 26 in theillustrated embodiment, such features can be predominately orexclusively formed in lower peripheral edge 44 of cover piece 22 infurther embodiments.

Addressing first hardstop features 96, 98, hardstop feature 96 is formedon an outer flat region 108 of lower peripheral edge 44 of cover piece22; the term “flat,” as appearing herein, referring to a region having asubstantially planar topology and extending principally in a planeorthogonal to package centerline 35 (FIG. 1). Comparatively, hardstopfeature 98 is formed in an outer peripheral portion of upper peripheraledge portion 38 of molded package body 26. In this example, hardstopfeature 98 assumes the form of a raised lip or rim, which projectsupwardly from the bulk of upper peripheral edge portion 38 and bounds anouter peripheral portion of annular channel 100. As indicated in FIG. 12by arrow 106, hardstop features 96, 98 are brought into physical contactwhen cover piece 22 is properly positioned or seated over molded packagebody 26. Hardstop features 96, 98 thus cooperate to precisely set ordetermine the vertical spatial relationship between cover piece 22 andmolded package body 26, as taken package centerline 35 (FIG. 1).Relatedly, hardstop feature 98 sets the vertical height of annularchannel 100 when enclosed or covered by flat region 108 of cover piece22; identified in FIG. 13 by arrows H₁, as measured from the bottom ofannular channel 100 to the uppermost surface of upper peripheral edgeportion 38. In one embodiment, H₁ may range between about 25 μm andabout 250 μm, as taken along package centerline 35. In otherimplementations, H₁ may be greater than or less than the aforementionedrange.

With continued reference to FIGS. 12-13, annular channel 100 extendsfully around upper peripheral edge portion 38 of molded package body 26.When cover piece 22 is positioned over package body 26 and,specifically, when hardstop features 96, 98 are brought into intimatephysical contact or abutment, annular channel 100 is enclosed by flatregion 108 of cover piece 22 and defines a volume of space or a voidhaving a fixed, predetermined volumetric capacity. Annular channel 100is desirably filled, in its entirety or its substantial entirety, with abonding material during cover piece attachment. Accordingly, acontrolled volume of bonding material may be dispensed or otherwiseapplied onto flat region 108 of cover piece 22 (shown), over annularchannel 100, or a combination thereof; the bonding material identifiedin FIG. 12 by reference numeral “46” as the bonding material is not yettransformed into its final shape and may still require curing, in someimplementations, to yield finalized cover bond layer 46 shown in FIG.13.

As the volumetric capacity of annular channel 100 is known, bondingmaterial 46′ can be provided in a volume that substantially matches orslightly exceeds the capacity. In this regard, in one implementation,bonding material 46′ may be dispensed or otherwise applied in apredetermined volume exceeding the volumetric capacity of annularchannel 100, while being less than the cumulative void or cavity spacebetween lower peripheral edge 44 of cover piece 22 and upper peripheraledge portion 38 of molded body 26 (equivalent to the cross-sectionalarea defined or bounded by features 96, 98, 100, 102, 104, 110, asextended in three dimensional space around the entire periphery of ACMpackage 20). In another embodiment in which annular channel 100 has avolumetric capacity of V_(C), while the bonding material is applied in apredetermined volume V_(BM), the following equation may apply:V_(C)<V_(BM)<2V_(C). Complete filling of annular channel 100 can betherefore ensured during cover piece attachment to enable the consistentand reliable formation of a continuous, high integrity 360° seal aroundthe periphery of air cavity 30. Additionally, as shown in FIG. 13, astep feature 110 may also be provided adjacent annular channel 100opposite hardstop feature 98 to bound an inner peripheral portion ofchannel 100, while having a height less than that of feature 98 toprovide an overflow area in fluid communication with channel 100 toaccommodate excess bonding material outflow. Thus, step feature 110 andpossibly contact walls 102, 104 may form a bonding material overflowreservoir, in a general sense; and, thus, may be contacted by thebonding material or cover bond layer 46 in the completed version of ACMpackage 20, as generally shown in FIG. 13. Comparatively, hardstopfeature 98 may serve as a damn or blockade feature, which preventsundesired outflow of the bonding material from the package interior andwhich is likewise contacted by cover bond layer 46 in the completedpackage.

In addition to or in lieu of the features described above, thecover-body interface may also be usefully imparted with one or moreangled contact surfaces, which help pilot or guide cover piece 22 intoproper position over molded package body 26. Two such angled contactsurfaces 102, 104 are shown in the example of FIGS. 12-13. As can beseen, angled contact surface 102 is formed on an inner peripheralportion of lower peripheral edge 44 of cover piece 22, while angledcontact surface 104 is formed on upper peripheral edge portion 38 ofmolded package body 26. Angled contact surfaces 102, 104 may or may notextend fully around the periphery of ACM package, as taken about packagecenterline 35 (FIG. 1). Contact surfaces 102, 104 are “angled” in thesense that surfaces 102, 104 do not extend principally within horizontalplanes orthogonal to package centerline 35. Instead, as labeled in FIG.12, angled contact surfaces 102 forms a first acute angle θ₁ with ahorizontal plane or flange frontside 42, as measured in a clockwisedirection; while angled contact surfaces 102 forms a second acute angleθ₂ with a horizontal plane or flange frontside 42, as measured in theclockwise direction. Angles θ₁, θ₂ are tailored to guide cover piece 22toward its properly centered over molded package body 26 as varyingregions of surfaces 102, 104 come into contact during the coverattachment process. The values of angles θ₁, θ₂ will vary amongstembodiments; however, in certain embodiments, and as indicated in FIG.12, θ₂ may exceed θ₁ such that angled contact surface 104 of cover piece22 is closer to perpendicular with respect to flange frontside 42 (FIGS.1, 2, 6, 9, and 11) than is angled contact surface 102 of molded packagebody 26. Angled contact surfaces 102, 104 are usefully located inboardof (that is, closer to the package centerline than are) hard stopfeatures 96, 98 and annular channel 100, although this need not be thecase in all implementations. Through the provision of such features,proper cover piece positioning and the formation of a high integritybond between cover piece 22 and molded package body 26 can beconsistently ensured across repeated iterations of the ACM packagefabrication process. This is highly desirable.

CONCLUSION

There has thus been provided ACM packages and methods for producing ACMpackages that can be carried-out in a relatively efficient, consistent,and cost-effective manner. Generally, such manufacturing processes mayinvolve the formation of molded package bodies around leadframes andbase flanges, which are staked or otherwise mechanically joined prior tothe molding process to provide precision alignment between the leadframeleads and the base flanges. In certain embodiments, the molded packagebodies may be formed to encompass or envelop the locating features ofthe base flanges (e.g., the above-described retention or staking posts),as well as corresponding locating features of the leadframes (e.g.,retention tabs having openings through which the retentions posts arereceived). Additionally, each molded package body may be formed toinclude an open cavity through which a device mount area of the baseflange is exposed. At least one microelectronic device is subsequentlyattached to the device mount area of the base flange, appropriateinterconnections are formed (e.g., by wirebonding), and a lid or coverpiece is bonded to the upper rim or edge of the molded package body tosealingly enclose the air cavity and complete fabrication of the ACMpackages. Device attachment is usefully performed utilizing a lowtemperature sintering process to create a robust, metallurgical bond atthe device-flange interface and to reduce undesired heat exposure of themolded package body.

In certain embodiments, the ACM package may further contain an optimizedcover-body interface formed between the lower peripheral edge of coverpiece and the upper peripheral edge or rim of the molded package body onwhich the cover piece seats. The optimized cover-body interface mayinclude physical features guiding precision alignment of the cover pieceduring cover piece attachment and/or which help promote the reliableformation of a high integrity, peripherally-continuous, low leakage bondat the cover piece-package body juncture. By way of example, suchphysical features can include: a fixed-volume bonding materialreservoir, which can be filled with a corresponding volume of epoxy oranother bonding material; raised hardstop features, which set thevertical height of the bonding material reservoir when the cover pieceis properly positioned over the molded package body; and/or angledcontact surfaces, which physically guide the cover piece into thedesired horizontally-centered position over the molded package body. Infurther embodiments, of the ACM package the cover-body interface mayinclude only a subset of the aforementioned features, may includedifferent features enhancing cover piece bonding, or may be formed tolack such features altogether.

In various embodiments, the above-described molded air cavity packageincludes a base flange, a molded package body bonded to the base flange,and package leads extending from the molded package body. The baseflange includes, in turn, (i) a flange frontside having a device mountarea; and (ii) a flange backside opposite the flange frontside, as takenalong a centerline of the molded air cavity package. Retention posts areintegrally formed with the base flange and extend from the flangefrontside in a direction opposite the flange backside. The retentiontabs have openings or apertures through which the retention posts arereceived. The molded package body may further be formed to envelope theretention posts and the retention tabs, in whole or in part. The packageleads and the retention tabs may comprise, for example, singulatedportions of a leadframe. Further, in implementation in which theretention posts extend substantially parallel to the centerline of theair cavity package, the retentions posts comprise deformed terminal endspreventing disengagement of the retention posts from the retention tabsalong the package centerline in a direction opposite the base flange. Insuch implementations, the deformed terminal ends may assume the form ofcircumferentially-expanded staked heads of the retention posts.

In further embodiments, the molded air cavity package includes a moldedpackage body having an upper peripheral edge portion, an air cavityaround which the upper peripheral edge portion extends, and a coverpiece bonded to the upper peripheral edge portion to sealingly enclosethe air cavity. The cover piece has a lower peripheral edge portion,which cooperates with the upper peripheral edge portion to define acover-body interface. The cover-body interface includes an annularchannel extending around the cover-body interface, as taken about thepackage centerline. The cover-body interface also includes first andsecond hardstop features formed on the upper peripheral edge portion ofthe molded package body and on the lower peripheral edge portion of thecover piece, respectively. The first and second hardstop featurescontact or physically abut to determine a vertical height of the annularchannel, as taken along the package centerline. In certain cases, thefirst hardstop feature assumes the form of a raised annular rim or lip,which extends around the upper peripheral edge portion of the moldedpackage body and possibly bounds an outer perimeter of the annularchannel. Additionally or alternatively, the molded air cavity packagemay further contain an annular step feature formed adjacent the annularchannel and having a height less than that of the raised annular rim, astaken along the package centerline. Bonding material, which attaches thecover piece to the molded package body, may contact the annular stepfeature. Finally, in some cases, a knurled surface region may beprovided on the flange frontside and may at least partially underlie theannular channel, as taken along the package centerline moving from thecover piece toward the flange backside (as illustrated in the example ofFIGS. 1-13),

In further embodiments, a method for producing the molded air cavitypackage, includes the step or processes of obtaining a base flangehaving a flange frontside from which a plurality of locating featuresextends, positioning the base flange with respect to a leadframe suchthat the plurality of locating features are received through openings inthe leadframe and aligning the base flange to the leadframe, anddeforming the plurality of locating features to mechanically capture thebase flange against the leadframe. A molded package body is formedaround selected regions of the base flange and the leadframe. In certainembodiments, the method may further include the steps of: (i) selectingthe leadframe to comprise a plurality of leads, retention tabs throughwhich the openings are provided, and dam bars connecting the retentiontabs to the plurality of leads; and (ii) severing the dam bars afterforming the molded package body around selected regions of the baseflange and the leadframe. Additionally or alternatively, in animplementation in which the plurality of locating features assume theform of staking posts, the step of deforming may entail the usage of astaking tool to circumferentially-expand head portions of the stakingposts after insertion through the openings in the leadframe.

In yet further embodiments, the method for producing a molded air cavitypackage may include positioning a microelectronic device in an aircavity defined, at least in part, by a base flange and a molded packagebody, the molded package body enveloping retention posts integrallyformed with the base flange. After positioning the microelectronicdevice in the air cavity, the microelectronic device may be electricallyinterconnected (e.g., by wirebonding) with package leads bonded to themolded package body. After electrically interconnecting themicroelectronic device, a cover piece may be bonded over the moldedpackage body to sealingly enclose the air cavity. In certainimplementations, the method may also include the steps or processes ofapplying a metal particle-containing precursor material at an interfacebetween the microelectronic device and the base flange, and sinteringthe metal particle-containing sinter precursor material to produce asinter bond layer bonding the microelectronic device to the base flange.

Finally, in a still further embodiments, the method for producing amolded air cavity package includes placing a base flange in a desiredspatial relationship with respect to package leads contained in aleadframe. A molded package body is formed around selected regions ofthe base flange and the leadframe. The molded package body is formed toinclude an upper peripheral edge portion contacting the package leadsand peripherally bounding an air cavity. The method further includes thestep or process of, when forming the molded package body, imparting theupper peripheral edge with an annular channel extending around the aircavity, as taken about a centerline of the air cavity package. Incertain cases in which the annular channel has a fixed volumetriccapacity, as measured when a cover piece is positioned over and contactsa hardstop feature provided on the upper peripheral edge portion of themolded package body, the method may further include the step or processof attaching the cover piece to the molded package body utilizing abonding material applied at an interface between the upper peripheraledge portion of the molded package body and a lower peripheral edgeportion of the cover piece. The bonding material is advantageouslyapplied in a predetermined volume exceeding the fixed volumetriccapacity, while being less than a cumulative void space between thelower peripheral edge portion of the cover piece and the upperperipheral edge portion of the molded package body.

While at least one exemplary embodiment has been presented in theforegoing Detailed Description, it should be appreciated that a vastnumber of variations exist. It should also be appreciated that theexemplary embodiment or exemplary embodiments are only examples, and arenot intended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the foregoing Detailed Description willprovide those skilled in the art with a convenient road map forimplementing an exemplary embodiment of the invention. It beingunderstood that various changes may be made in the function andarrangement of elements described in an exemplary embodiment withoutdeparting from the scope of the invention as set forth in the appendedclaims.

1. A molded air cavity package, comprising: a base flange, comprising: aflange frontside having a device mount area; and a flange backsideopposite the flange frontside, as taken along a centerline of the moldedair cavity package; retention posts integrally formed with the baseflange and extending from the flange frontside in a direction oppositethe flange backside; retention tabs having openings through which theretention posts are received; a molded package body bonded to the baseflange and enveloping, at least in substantial part, the retention postsand the retention tabs; and package leads extending from the moldedpackage body.
 2. The molded air cavity package of claim 1 wherein thepackage leads and the retention tabs comprise singulated portions of aleadframe.
 3. The molded air cavity package of claim 1 wherein theretention posts extend substantially parallel to the centerline of theair cavity package; and wherein the retentions posts comprise deformedterminal ends preventing disengagement of the retention posts from theretention tabs along the package centerline in a direction opposite thebase flange.
 4. The molded air cavity package of claim 3 wherein thedeformed terminal ends comprise circumferentially-expanded staked headsof the retention posts.
 5. The molded air cavity package of claim 1wherein the retention tabs comprise singulated surfaces visible from anexterior of the molded package body.
 6. The molded air cavity package ofclaim 1 further comprising punched depressions formed in the flangebackside opposite the retention posts, as taken along the centerline ofthe molded air cavity package.
 7. The molded air cavity package of claim1 wherein the base flange further comprises a texturized surface regioncovered by and bonded to the molded package body.
 8. The molded aircavity package of claim 7 wherein the texturized surface region islocated adjacent junctures formed between the retention posts and theflange frontside.
 9. The molded air cavity package of claim 1 furthercomprising: a microelectronic device contained in the air cavity packageand electrically interconnected with the package leads; and a sinteredmetallic bond layer attaching the microelectronic device to the devicemount area of the base flange.
 10. The molded air cavity package ofclaim 9 wherein the sintered metallic bond layer is predominatelycomposed of at least one of the group consisting of silver, gold, andcopper, by weight.
 11. The molded air cavity package of claim 1 whereinthe package leads comprise proximal end portions bonded to the moldedpackage body; and wherein the molded package body comprises at least onelead isolation shelf located between the flange frontside and theproximal end portions of the plurality of leads, as taken along thecenterline of the molded air cavity package.
 12. The molded air cavitypackage of claim 11 wherein the molded package body further comprises anupper peripheral edge portion integrally formed with the lead insolationshelf, contacting the package leads, and bounding an outer periphery ofan air cavity provided in the molded package body.
 13. The molded aircavity package of claim 1 further comprising: an air cavity; amicroelectronic device located in the air cavity, bonded to the baseflange, and electrically interconnected with the package leads; and acover piece bonded to the molded package body to sealingly enclose theair cavity.
 14. A method for producing a molded air cavity package,comprising: obtaining a base flange having a flange frontside from whicha plurality of locating features extends; positioning the base flangewith respect to a leadframe such that the plurality of locating featuresare received through openings in the leadframe and aligning the baseflange to the leadframe; deforming the plurality of locating features tomechanically capture the base flange against the leadframe; and forminga molded package body around selected regions of the base flange and theleadframe.
 15. The method of claim 14 further comprising: selecting theleadframe to comprise a plurality of leads, retention tabs through whichthe openings are provided, and dam bars connecting the retention tabs tothe plurality of leads; and severing the dam bars after forming themolded package body around selected regions of the base flange and theleadframe.
 16. The method of claim 14 wherein the plurality of locatingfeatures comprises staking posts, and wherein the deforming comprisesutilizing a staking tool to circumferentially-expand head portions ofthe staking posts after insertion through the openings in the leadframe.17. The method of claim 14 further comprising selecting the base flangeto have knurled surface region extending at least partially around anouter peripheral portion of the flange frontside.
 18. The method ofclaim 14 further comprising: forming the molded package body to have anupper peripheral edge portion, which bounds a peripheral portion of anair cavity; and bonding a cover piece to the peripheral rim portion tosealingly enclose the air cavity.
 19. A method for producing a moldedair cavity package, comprising: positioning a microelectronic device inan air cavity defined, at least in part, by a base flange and a moldedpackage body, the molded package body enveloping retention postsintegrally formed with the base flange; after positioning themicroelectronic device in the air cavity, electrically interconnectingthe microelectronic device with package leads bonded to the moldedpackage body; and after electrically interconnecting the microelectronicdevice, bonding a cover piece over the molded package body to sealinglyenclose the air cavity.
 20. The method of claim 19 further comprising:applying a metal particle-containing precursor material at an interfacebetween the microelectronic device and the base flange; and sinteringthe metal particle-containing sinter precursor material to produce asinter bond layer bonding the microelectronic device to the base flange.